The present invention relates to a method of fabricating a fiber optic coupler by fusion stretching techniques.
According to a conventional method employing fusion stretching techniques, a fiber optic coupler having a desired splitting ratio at a specified wavelength is produced by fusion stretching two optical fibers of the same propagation constant, but the splitting ratio of such a fiber optic coupler has substantial wavelength dependence. It is therefore difficult, with the prior art method, to fabricate a fiber optic coupler having a splitting ratio over a desired wide wavelength range. One method that has been proposed to solve this problem by fusion stretching two optical fibers of different propagation constants is disclosed in, for example, U.S. Pat. No. 4,798,438. With reference to its fabrication process shown in FIG. 1 the conventional method, comprises the steps of:
(A) Providing two optical fibers 11 and 14 of the same diameter and the same propagation constant, cut off from the same optical fiber, and heating the optical fiber 11 along its longitudinal segment by burners 12 and 13 while stretching the optical fiber 11 in opposite directions to reduce the diameter of the longitudinal segment to thereby change its propagation constant;
(B) Holding the thus stretched optical fiber 11 and the other fiber 14 in parallel juxtaposition with each other;
(C) Fusing together the optical fibers 11 and 14 along their longitudinal sections by the burners 12 and 13; and
(D) Stretching the optical fibers 11 and 14 by applying thereto equal tension.
FIGS. 2 through 4 show the wavelength vs. splitting ratio characteristics of fiber optic couplers 15 fabricated according to the conventional method, using optical fibers 125 .mu.m in outer diameter before they were stretched.
The splitting ratio of the fiber optic coupler is defined as follows: EQU Splitting ratio=P.sub.4 /(P.sub.1 +P.sub.4).times.100(%)
where P.sub.1 and P.sub.4 are energies of light emitted from one end of, for example, the optical fiber 14 supplied at the other end with light and from one end of the other optical fiber 11 on the same side as the above-said one end of the fiber 14.
FIG. 2 shows the case of a fiber optic coupler produced by fusing together and stretching the optical fibers 11 and 14 of the same diameter without involving step A. As seen from FIG. 2, the splitting ratio of this fiber optic coupler is zero at a specific wavelength. By increasing the stretching length in step D, the wavelength characteristic of the splitting ratio is shifted toward the short wavelength side as indicated by the arrow. This property is common to the cases of FIGS. 3 and 4.
FIG. 3 shows the case of a fiber optic coupler fabricated involving step A in which the optical fiber 11 was stretched to reduce the outer diameter D.sub.1 of its longitudinal section to 96 .mu.m. A minimum value of the splitting ratio is 13% in this example.
FIG. 4 shows the case of a fiber optic coupler fabricated involving step A in which the optical fiber 11 was stretched to reduce the outer diameter D.sub.1 of its longitudinal section to 84 .mu.m. A minimum value of the splitting ratio is 64% in this example.
By stretching the one optical fiber 11 in step A to make its propagation constant different from that of the other optical fiber 14 as mentioned above, it is possible to fabricate a fiber optic coupler whose splitting ratio has a desired value, and the splitting ratio undergoes no substantial change with wavelength in the vicinity of the wavelength at which the splitting ratio is minimum, and consequently, a relatively wide-band fiber optic coupler can be implemented. In addition, the center wavelength at which the splitting ratio is minimum can be shifted by changing the amount the fiber is stretched in step D. Further, the fact that in each case of FIGS. 2, 3 and 4 the minimum splitting ratio remains substantially constant in spite of increased stretching length in step D suggests that the difference in propagation constant between the two optical fibers 11 and 14 is kept substantially constant.
Comparison of FIGS. 3 and 4 reveals that the minimum value of the splitting ratio will markedly change from 13% to 64% even with a slight change in the outer diameter of the optical fiber 11 in step A. In the case of manufacturing a fiber optic coupler whose splitting ratio has a minimum value as close to 50% as possible at wavelengths in the range of 1.1 to 1.6 .mu.m, the outer diameter D.sub.1 of the optical fiber 11 in step A must be selected a little greater than 84 .mu.m.
With the above-described conventional manufacturing method, the minimum value of the splitting ratio of the fiber optic coupler depends essentially upon the outer diameter of the reduced-diameter section produced by stretching the longitudinal section of the one optical fiber 11 in step A. However, the outer diameter of the reduced-diameter section scatters considerably among specimens, and even if the stretching length or time is adjusted in step D, the splitting ratio is merely shifted toward the axis of wavelength, so that the minimum value of the splitting ratio cannot be controlled. On this account, the minimum value of the splitting ratio of the fiber optic coupler scatters largely, resulting in the yield rate of product being very low.